1. Field of the Invention
The present invention relates to a hexagonal ferrite magnetic powder, a method for producing the same and a magnetic recording medium. More particularly, it relates to a hexagonal ferrite magnetic powder which enables one to reduce noise without reducing as and which is adapted for a magnetic recording medium for high-density recording reproducible by a highly sensitive head such as an MR head or a GMR head. Also, the invention relates to a magnetic recording medium containing the hexagonal ferrite magnetic powder in the magnetic layer.
2. Description of the Related Art
In the field of magnetic recording, a magnetic head based on the principle of electromagnetic induction (induction type magnetic head) has been used and spread. However, the induction type magnetic head is becoming insufficient for use in the presently required reproduction of high-density records. That is, when the number of coil turns of a reproduction head is increased in order to obtain a larger reproduction output, there results an increased inductance and an increase in resistance in high frequency region, leading to a problem of reduction in reproduction output. Thus, in recent years, a reproduction head based on the principle of MR (magnetic reluctance) has been proposed and used for a hard disc or the like. The MR head yields a reproduction output several times as much as that of the induction type magnetic head and, since no induction coils are used, device noises such as impedance noise are markedly reduced. Therefore, a large SN ratio can be obtained.
On the other hand, the improvement of high-density recording characteristics can also be made by reducing magnetic recording medium noise having conventionally been hidden behind the device noises.
In order to attain such object, there has been proposed, for example, a magnetic recording medium comprising a non-magnetic support having provided thereon a magnetic layer containing a hexagonal ferrite magnetic powder dispersed in a binder (see, for example, JP-A-10-312525).
Also, improvement of the hexagonal ferrite magnetic powder is disclosed in JP-A-56-169128, JP-A-58-169902 and JP-A-10-92620.
However, the above-mentioned related art fails to achieve noise reduction without reducing as and fails to provide a magnetic recording medium for high-density recording currently required.
It can be considered to reduce particle size of the magnetic powder for the purpose of reducing noise of the magnetic recording medium. In general, however, reduction of the particle size of the magnetic powder leads to reduction of σs. With hexagonal ferrite, too, reduction of the particle size of the magnetic powder causes reduction of σs. Also, in order to obtain a high SN ratio, low noise and high output are necessary. However, reduction of σs of the magnetic powder causes reduction of output on the other hand, with a magnetic recording medium for use in high-density recording, it is necessary to reduce the thickness of the magnetic layer in order to reduce thickness loss and reduce. PW50.
However, since the thickness of the magnetic layer is rendered small and σs of the magnetic powder is reduced, the output becomes smaller, thus sufficient performance not being obtained. In addition, low noise and high output are required for a magnetic recording medium upon conducting high-density recording.